1. Field of the Invention
The present invention relates to a stepping motor control system used in a disk drive unit for moving a head in the seek operation.
2. Description of the Prior Art
In a disk drive unit, when accessing to desired one of tracks concentrically formed on a disk such as a floppy disk which is placed on a turntable for rotation therewith, a seek operation is performed to move a head up to the desired track in the radial direction of the disk at a high speed. The head is mounted on a head carriage and a needle (drive pin) attached to the head carriage engages a lead screw rotatable by a stepping motor. When the lead screw is rotated by the stepping motor, the needle moves in the axial direction of the lead screw, whereby the head carriage and hence the head are moved in the radial direction of the disk.
In such a disk drive unit, the stepping motor is rotated in response to a command from the host side. The command is given in the form of pulses (step pulses). For each step pulse, the stepping motor rotates to such an extent that the head is moved by one track pitch. Accordingly, when step pulses are successively sent, the stepping motor rotates by a predetermined angle per pulse so that the head is moved in a one-track by one-tack basis. A seek speed of the head is determined by the cycle of the step pulses. While the pulse cycle may be set to various values, the drive unit for floppy disks sets about 3 msec as the shortest cycle of the step pulses and employs cycles of about 3 msec, about 6 msec, about 12 msec, etc. The host side sends the step pulses of various cycles, whereupon the disk drive unit exhibits a seek speed different for each cycle.
FIG. 4 shows changes in a head position during the seek operation.
In FIG. 4, when now accessing to a track with its center indicated by a one-dot-chain line, the head is moved with the seek operation along a locus, indicated by a solid line a, to be drawn into the target track. However, because of undergoing an external load, a stepping motor can no longer be driven in an angular range where the torque becomes smaller than a predetermined value. This range is called a dead zone and illustrated as locating on a disk in FIG. 4. When the head is moved along the solid line a and comes into the dead zone, the stepping motor loses its driving force and the head goes beyond the dead zone due to inertia of the head carriage and the stepping motor. This causes the stepping motor to produce a driving force in a direction of reversal of the above step from the relationship between magnetic poles and excitation phase signals of the stepping motor, whereby the head is moved back toward the center of the target track. The head therefore enters the dead zone, but goes beyond the same again, causing the stepping motor to produce a driving force in a direction of reversal of the above step. In this way, as indicated by a solid line b, the head position oscillates while going into and out of the dead zone, and the amplitude of the oscillation gradually damps so that the head position is finally stabilized in the dead zone. Such oscillation is called settling. The occurrence of settling gives rise to problems as follows.
(1) When the head intersects the track during the seek operation, there occurs seek noise which generates undesired noisy sounds.
Now taking a drive unit for floppy disks as an example, the drive unit is set such that when the seek operation is performed by supplying the step pulses with the shortest cycle of about 3 msec, the locus of head movement in the vicinity of the track follows the solid line a and then a broken line c in FIG. 4 without causing any settling. On the contrary, when the seek operation is performed by supplying the step pulses with the cycle of about 6 msec, because of the step pulse cycle being longer than about 3 msec, the head approaches the track along the solid line a, but upon going beyond the track, it follows the solid line b to cause the settling. The next step pulse is supplied during the settling and, therefore, the head now moves along a broken line d. When the head is shifted from the locus of the solid line b to the locus of the broken line d upon supply of the step pulse, the stepping motor produces a so large driving force that the direction of movement of the head is abruptly reversed to thereby cause seek noise.
Heretofore, there is known a technique adapted to prevent noise which occurs when the rate of step pulses is almost equal to the natural oscillation frequency of a stepping motor. With this technique, by always monitoring the cycle of step pulses, when the cycle is not included in a range corresponding to the natural frequency, excitation pulses of A and B phases for the stepping motor are normally controlled; i.e., the polarity of the A-phase pulse is reversed at the timing of each step pulse and the polarity of the B-phase pulse is reversed with a delay of certain time t from the timing of each step pulse. Thus, the stepping motor is excited in a normal manner. On the other hand, when the step pulse cycle is included in the above range, a reverse excitation phase pulse having opposite polarity to the A-phase pulse (namely, in a direction of reversely rotating the stepping motor) is added to the A-phase pulse at the timing of a peak of the solid line b in FIG. 4 between the timing of each step pulse and the reversal timing of the polarity of the B-phase pulse, and a reverse excitation phase pulse having opposite polarity to the B-phase pulse (namely, in a direction of reversely rotating the stepping motor) is added to the B-phase pulse at the timing of a next peak of the solid line b in FIG. 4 after reversal of the polarity of the B-phase pulse. Adding the reverse excitation phase pulses as mentioned above implies that the step pulse cycle is essentially divided by the reverse excitation phase pulses. As a result, the step pulse cycle departs away from the natural oscillation frequency of the stepping motor to reduce a level of noise.
However, the above prior art requires it to always monitor the cycle of step pulses and perform a process of adding or not the reverse excitation phases pulse to the excitation phases depending on the cycle, thus making the process complicated.
Also, in the above prior art, the reverse excitation phase pulses are added with an intention of braking the stepping motor to suppress the amplitude of the settling. Even by so controlling, however, when the head is moved along the solid line a and then the broken line d shown in FIG. 4 for seeking, the seek noise produced upon shifting of the head from the solid line b to the broken line d cannot be suppressed sufficiently.
(2) When accessing to a desired track, there occurs such hysteresis that the head is stopped at different positions in the direction of width of the track depending on the direction of seeking (i.e., the direction of movement of the head).
When the head is drawn into the desired track after the seek operation, the head is stopped after being subjected to settling as represented by the solid line b in FIG. 4. At this time, owing to the load applied to a rotor of the stepping motor, the head is not always stopped such that its center aligns with the center of the track, and both the centers are not coincident with each other in usual cases. Supposing now in FIG. 5 that the head is moved toward a desired track nTK to be accessed along a solid line a from a track (n-1)TK on one side, the head is stopped after the above-mentioned settling. Conversely, supposing that the head is moved toward the desired track nTK along a solid line a' from a track (n+1)TK on the other side, the head is also stopped after the settling as shown. It is however general that the centers of the heads thus moved and stopped do not align with the center of the track nTK and take arbitrary positions in the dead zone, meaning that the head stop positions are not coincident with each other. This phenomenon is called hysteresis. With the presence of such hysteresis, the head stop positions are offset from the track center and also from each other. For a floppy disk of 3.5 inch, the dead zone amounts to 20 to 30% of width of each track and, therefore, the head exhibits off-track on the same order in the worst case. The off-track amount is further increased depending on expansion of disks due to changes in temperature and service life of the stepping motors.
To eliminate the hysteresis, the following technique is proposed in the prior art. When the head is moved toward the desired track nTK in one direction as indicated by the solid line a in FIG. 5, it is stopped like conventionally, namely, as indicated by the solid line a in FIG. 5. On the other hand, when the head is moved toward the desired track nTK in the opposite direction as indicated by the solid line a' in FIG. 5, the stepping motor is further driven to move by another track pitch after reaching the desired track nTK. Upon the head being stopped in the adjacent track, the stepping motor is now driven in a direction of reversal to the above, causing the head to be drawn into the desired track. Thus, irrespective of the direction of seeking, the head is finally drawn into the desired track along the solid line a in FIG. 5 and the head stop positions are coincident with each other.
With that conventional technique, however, when the head is moved in the opposite direction for seeking, an extra time is necessary for the head to reciprocate one track pitch, which prolongs an access time. Some hosts require read/write immediately after sending the step pulses corresponding to the seek distance. Such requirement in those hosts cannot be met if the head is moved to reciprocate one additional track pitch as stated above.
Meanwhile, as also shown in FIG. 5, when the seek operation of the carriage is performed by using the stepping motor with the step pulses SP of short cycle, the locus of head movement in the vicinity of the track follows the solid line a and then a broken line b without causing any settling. On the contrary, when the seek operation of the carriage is performed with the step pulses of long cycle, the head approaches the track along the solid line a, but upon going beyond the track, it follows a solid line d to cause the settling. The next step pulse is supplied during the settling and, therefore, the head now moves along a solid line c. When the head is shifted from the locus of the solid line d to the locus of the solid line c, the stepping motor produces a so large driving force that the direction of movement of the head is abruptly reversed to thereby cause seek noise.
As explained above, the conventional system for controlling a stepping motor suffers from the problems of causing hysteresis in the head stop positions with respect to a target track and producing seek noise upon supply of a step pulse during the settling. In particular, with the stepping motor having small size and hence small inertia, seek noise generated in the seek operation may be increased because of a resonance phenomenon during the driving of the carriage. This is likely to occur when the interval of step pulses is almost equal to the natural oscillation frequency of the stepping motor or the frequency of a fraction thereof.